Carbohydrates Flashcards
Carbohydrates are
- Important structural components
- important component of nucleic acids
- contribute to protein structure
What is a peptidoglycan?
- forming the cell wall
- Prokaryotes
- cross linking peptides, mesh network
What are proteoglycans?
- Eukaryotes
- Form extracelluar matrix, which form bulk around cells to protect
Monosaccharides, disaccharides and polysaccharides.
Mono - Fruits, veg, honey, nuts
Dis - Sugars, milk
Poly - Rice, potatoes, corn, wheat
Sugar is made up of
- glucose and fructose
- once glucose transporters are saturated, the body cannot absorb any additional sugar in the form of glucose .
- due to transport mechanisms to pass through intestinal wall
- fructose diff mech
Simple sugars are
- either aldoses or ketoses
Glucose and galactose are
Aldoses
Fructose is a
Ketose
Aldoses contain
Aldehyde group
Ketoses contain
Ketone group
D-Glyceraldehyde and L-glyceraldehyde
Enantiomers, form mirror images due to chiral centres
Optical Isomerism
- What happens when we shine light through a polarising filter?
Turn detector to left to get max light - l
Turn to Right - d
Glucose formed by
Photosynthesis
Fisher Convention/Projection
- Worked out Glyceraldehyde had two forms
- D and L
- D OH on right
If a molecule has N chiral centres
2 to the power of N isomers (bel Van hoff rule)
In Isomers of Aldo-tetroses we call mirror images
Erythrose and Threose (D and L)
Erythro refers to
- functional groups on same side of molecule
Aldehyde react with alcohol to form
- hemiacetal
Ketone with alcohol to form
- Hemiketal
How glucose forms into a ring
- 2 possible conformations depending on which way round you form the ring
- Alpha or Beta anomer
1) OH-5 reacts with C1 aldehyde
2) Forms hemiacetal of carbon no 1 position
3) ring structures are reversible
Glucose can form 2 diff rings known as
6 membered is Pyranose 5 membered is Furanose (alpha and beta anomer of each) C1 position OH down in alpha up in beta - changes stability of 3D structure of rings
Beta form are more/less stable?
- more stable
Two anomers of glucose called…
-pyranose and furanose
2 other examples of hexoses that are metabolised
- galactose and mannose
- these are epimers of glucose
An epimer is a
Chirally opposite centre
Stability of diff anomers in free solutions varies
- in manose its alpha which is more stable than beta
Stability can be measured by seeing
optical rotation over time
Most stable form of these enantiomers
- Chair, can flip to boat though
Fructose features
- forms a 5 carbon ring, can only form a furanose ring due to carbonyl in position 2
- Beta and Alpha anomer
ATP features
- Structure, 5 membered sugar ring, 3 phosphate groups on end.
- phosphate bonds are thermodynamically unstable
- High energy phosphate bonds
- kinetically stable though
Hydrolysis of ATP
- yield energy
- converted to ADP and Pi
- 4 oxygen groups neg charge disruputed around phosphate ion, gives stability
Standard conditions hydrolysis of ATP
- room temp and pressure
- 1 mol conditions
- 30KJ/Mol of ATP
- Hydrolyse to pyrophosphate
ATP acting as currency
positioned in between high and low energy levels
Constantly making ATP done by intermediary metabolism
- breaking down in small steps
Glycolysis
1) Glucose –> 2 pyruvate
2) must put in ADP
3) and NAD cofactors
4) Pyruvate has 2 fates, go on to further oxidation or can produce lactate
2 ways in which glucose enters body
Facilitated diffusion transporters - GLUT
Sodium linked active transporters - SGLT
GLUT 2 transporters allows
Both glucose and fructose into cells
Insulin use…
- control expression on cell surface of GLUT 4 transporter
- short term it will enable increase in transport
- long term changes in glucose metabolising enzymes
Once glucose is inside the cell glycolysis happens…
1) Glucose acted on by hexokinase (PHOSPHOTRANSFER)
2) Base catalysed isomerization -Phosphohexose isomerase - glucose-6-phosphate switches to fructose-6-phosphate
3) Phosphofructokinase sticks another phosphate onto molecule. Fructose 6 - phosphate –> Fructose 1,6 - bisphosphate.
4) Adolase reaction, cleaves 1,6-bisphosphate into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (2 3C compounds formed)
5) Isomerisation by Triose Phosphate Isomerase. Dihydroxyacetone phosphate goes to Glyceraldehyde 3-Phosphate
6) Glyceraldehyde 3-Phosphate Dehydrogenase converts Glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate.
Transfer of proton.
7) Phosphoglycerate kinase then transfers 1,3-bisphospoglycerate to 3-Phosphoglycerate.
8) Phosphoglycerate mutase catalyses 3-Phosphoglycerate to 2-phosphoglycerate
9) Enolase reduces 2-phosphoglycerate to Phosphoenolpyruvate
10) Pyruvate Kinase - ATP reacts with phosphate group to form another molecule of ATP and form phosphoenolpyruvate (unstable intermediate) then isomerises into pyruvate
Hexokinase does
- transfers phosphate group onto C6 on glucose via ATP (ATP consuming reaction)
- requires magnesium
(cofactor) - nucleophilic attack from glucose to phosphate
Aspartate residue does what in phosphotransfer?
destabilises OH group on C6 of glucose, allows OH to be more nucleophilic
Hexokinase types and features
1,2,3 have high affinity, allosterically regulated bu G6P and ATP
- Glucokinase Hexokinase IV has low affinity
- example of induced fit model (excludes H2O from active clef)
Evolution of Hexokinase lead to what structure
- Mammalian HKs have a duplicated domain structure
- one is regulatory domain, similar structurally
- catalytic domain
- AMP - relieve inhibitory affect of ADP when binding
Liver Glucokinases
- difference bwteen GK and HK 1-3 is that GK works at high glucose conc
- low affinity
Base catalysed isomerization
- ring form broken
- can happen by lysine taking a proton, get open chain which can be rearranged
- enzyme which does this also has functions in signalling in immune system
Chemical affect of Phosphate group
- Make molecule charged (negative)
- phosphorus more electronegative
Two biochemical effects of glucose phosphorylation
The –ve charge prevents facultative diffusion out of the cell
The electron-withdrawing effect increases reactivity of the saccharide
Phosphofructokinase features
- ATP (substrate) able to inhibit reaction
- allosteric regulation of enzyme behaviour
- rate of reaction becomes dependent on conc of regulatory molecules, ATP also a coregulator
What does enzyme existing in tetromeric form allow?
- Flip from active to inactive state
- R state (active) stabilised by ADP binding at allosteric site
- If a lot of ATP then ADP displaced from allosteric site so will then be T state (inactive) active sites closed off.
- Citrate binds to allosteric site too to cause switch off
Citrate function…
- oxidative metabolism results in reduction of citrate
- citrate then acts as an allosteric regulator to switch off phosphofructokinase and reduce production of pyruvate from glucose
Phosphofructokinase exists in X isoforms?
Two
Aldolase mechanism in bacteria
- key amino acid which catalyses starts of with lone pair of electrons on lysine acting as a nucleophile to attack fructose 16 bisphosphate, form a Schiff base
- then allows for a ketoenol reformation
- assisted by acid residue.
- aspartate
Aldolase mechanism in Mammailian
- aspartate replaced by tyrosine
- catalytic lysine as a nucleophile
Aldolase general features
- homo-tetramer
Mechanism for Isomerisation stage 5
Endediol intermediate
- forms a transition state ^
Glyceraldehyde 3-Phosphate Dehydrogenase function in stage 6
catalyses reduction of NAD and transphosphorylation of inorganic phosphates to the sugar backbone.
NADH features
- mediates redox transfer in cell
- structure of it allows it to switch between oxidised and reduced forms
STAGE 6 Glyceraldehyde 3-Phosphate Dehydrogenase MECHANISM
- enzyme forms intermediate via the nucleophilic donation of a bond from a cysteine group to the aldehyde end group on molecule.
- then opens double bond and allows hydride ion to be transferred to NAD+ in reduction step.
- then rearranges to release product
STAGE 7 - Phosphoglycerate kinase then transfers 1,3-bisphospoglycerate to 3-Phosphoglycerate.
- enzyme folds in on itself to stop water coming in
- allows phosphate transfer in active clef without H2O
- get 2 ATP back so far
STAGE 8 - Phosphoglycerate mutase catalyses 3-Phosphoglycerate to 2-phosphoglycerate
- transfer phosphate from 3-2 position
- isomerase reaction
- using Mg2+ cofactor
- teteromeric enzyme
WHAT CLASS OF REACTION IS - Enolase reduces 2-phosphoglycerate to Phosphoenolpyruvate
- lyase reaction (hydro)
Phosphoenolpyruvate is…
energetically unfavourable structure
MECHANISM OF STAGE 9 Enolase reduces 2-phosphoglycerate to Phosphoenolpyruvate
- Metal ion cofactor dependent system
- taking electrons away from carbonyl group to anchor substrate
- allowing base catalysed rearrangement
- allows transfer of oxygen to a free proton to release water and enol
Enolase is a what enzyme
Dimer
Pyruvate Kinase is a what enzyme
Teteromeric
- allosteric sites for (fructose1,6-bisphosphate and ATP)
- They switch conformation state of enzyme from active to inactive
- ATP drops compared to ADP enzyme becomes active and allows production of more ATP
Pyruvate Kinase reaction points
- irreversible reaction due to pull by enol-keto isomerisation
- allosterically regulated by tetramer
- inhibited by fructose1,6-bisphosphate and ato
ATP YIELD AT END
- 2 molecules of ATP from Phosphoglycerate kinase reaction and 2 from pyruvate kinase reaction
- produce 4 but needed 2 to phosphorylate glucose-6-phosphate and fructose 1,6-bisphosphate
Anaerobic conditions - lactate/ end point of glycolysis
- pyruvate produces lactate
- produce lactate also in microbiological reactions
- microrganisms can produce ethanol
Aerobic conditons - fate of pyruvate
- metabolised to CO2 and H2O
Fermentation products
- use it for alcohol production
- bread and yoghurt
- Yoghurt produced by lactic fermentation
- production of antibiotics
How is NAD+ regenerated from NADH?
- reduction of pyruvate to lactate regenerates NAD+
Lactate dehydrogenase reaction
Pyruvate (ketone group) reduced to lactate (alcohol group)
Lactate dehydrogenase two forms
- muscle form - preference for pyruvate to lactate when low oxygen
- heart form - preference for lactate to pyruvate in high oxygen
In yeast
- ADH
- pyruvate–>ethanal–>ethanol +CO2
In humans is alcohol converted back to ethanal and NADH. There are 3 problems:
1) ethanal is toxic and damages proteins
2) excess NADH produced uses up NAD+
3) reduced gluconeogenesis from lactate in liver leading to hypoglycaemia
Pyruvate is a…
an alpha keto-carboxylic acid
Gluconeogenesis is…
- amino acids –> pyruvate
- fatty acids –> acetyl coA
- Lactate –> pyruvate
- all of which can be converted then back to glucose
Gluconeogenesis is in the liver…
- pyruvate enters mitochondria
- pyruvate –> oxaloacetate –> malate –> malate out of mitochondria, oxaloacetate –> PEP –> Fructose16-bisphosphate –> Fructose 6-phosphate –> Glucose 6-Phosphate, Glucose
3 irreversible steps in glycolysis
hexokinase G —-> G6P
phosphofructokinase F6P—-> FBP
pyruvate kinase PEP—-> pyruvate
Essentially 3 irreversible steps in glycolysis- Hence 3 bypass steps are required for gluconeogenesis:
Glucose 6 phosphatase G6P —-> G
Fructose bis-phosphatase F(1,6)BP —-> F6P
PEP carboxykinase + OxA—-> PEP
Pyruvate carboxylase pyruvate —-> OxA
Enzymes in Gluconeogenesis…
- Pyruvate carboxylase converts pyruvate (3C ) to oxaloacete (4C)
- Malate dehydrogenase converts oxaloacetate back to malate
- Malate Dehydrogenase converts Malate to Oxaloacetate
- PEP carboxykinase converts OAA to PEP
- Fructose Bisphosphatase converts Fructose1,6-bisphosphate to Fructose6-Phosphate
- Glucose 6-Phosphatase converts Glucose 6-to glucose
Gluconeogenesis is opposite of
Glycolysis
What does a check point do?
- need check point at beginning of anabolic reaction so that we don’t make substrate when its not necessary
Pyruvate Carboxylase does… IN MITOCHONDRIA
Pyruvate to Oxaloacetate by addition of CO2 and ATP, put carboxy group on 3rd C atom to make 4 Carbon
- one molecule of ATP to drive that reaction
- Glucose out of 2 pyruvates use 2 molecules of ATP for 2 OAA
Pyruvate coarboxylase activity can have an x FUNCTION???
anaplerosis function (replenishing mitochondrial oxaloacetate)
Pyruvate carboxylase is dependent on what?
- Biotin as a cofactor and magnesium ions
- it is a 4 domain enzyme
- teteromeric structure
Pyruvate Carboxylase is switched on by
- acetyl CoA
Pyruvate C domains
- carboxyl transferase domain
- allosteric linking domain
- biotin carrier domain
- biotin carboxylase domain
Acetyl CoA function in Pyruvate C
2 step
- CO2 forms complex with biotin
- complex transfers carboxyl group to pyruvate to form OAA
Transporter molecules in mitochondria membrane to allow
Mitochondria to transport Malate into cytosol (diffusion)
Formation of Phospho enol Pyruvate
- GTP coupled reaction
- OAA to PEP
Regulators for Phosphofructokinase
- Inhibitors ATP and citrate
- High level of citrate cell is in a high energy state
- AMP activator
- F(2,6)BP - activator
FBPhospatse regulators
- AMP - Inhibitor
- ATP, citrate - no effect
- F(2,6)BP - inhibitor
Glucagon and Insulin are major hormone regulators
- Glucagon leads to raising blood glucose
- Insulin leads to lowering blood glucose
Phosphofructokinase 2 - Insulin and Glucagon control this
- its a bifunctional enzyme
- has a kinase activity - activated by insulin
- phosphatase activity 0 phosphorylated and inhibited by glucagon
- regulate level of F26BP
- Insulin stimulate glycolytic pathway
- Glucagon will inhibit and stimulate gluconeogenic pathway
Structure of Phosphofructokinase 2
- Kinase and Phosphatase stuck together
Insulin action - Short term
Increase in glucose transport via the GLUT4 transporter
- Insulin receptor signalling stimulates GLUT4 exocytosis
- GLUT4 translocates through multiple intracellular compartments
Insulin Long Term effects
- Decreased protein synthesis of gluconeogenic enzymes eg PEP-CK
- Increased protein synthesis of glycolytic enzymes eg GK (glucokinase)
Aldotetrose are unable to…
to form stable ring structures
Phosphohexose isomerase mechanism involves an…
enediol intermediate
